1. Field
The present technology relates to coupling an electronic component to a printed circuit board.
2. Description of the Related Art
The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
While a wide variety of packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted on a substrate. The substrate may in general include a rigid, dielectric base having a conductive layer etched on one or both sides. Electrical connections are formed between the die and the conductive layer(s), and the conductive layer(s) provide an electric lead structure for connection of the die to a host device. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a molding compound to provide a protective package. SiP and MCM packages may be made as BGA (ball grid array) packages including solder balls affixed to contact pads on a lower surface of the die. These packages may alternatively be made as LGA (land grid array) packages, which have lands, or contact fingers, without solder balls on the lower surface.
Electronic components such as BGA and LGA packages may be surface mounted to a printed circuit board (PCB) as part of an electronic system. The solder ball array of the surface mounted electronic component may be placed on a portion of the PCB having a like configuration of contact pads, and then a reflow process may be performed to melt the solder balls and couple the surface mounted electronic component to the PCB.
FIG. 1 shows an example of a BGA package 20 mounted to a PCB 22 via an array of solder balls 24. The BGA package includes a substrate 26 and a protective mold cap 28 housing one or more semiconductor die. FIG. 2 is a bottom perspective view of the BGA package 20 including the array of solder balls 24. The size and configuration of solder balls is by example only, and it is understood that there may be a greater or lesser number of solder balls 24, and they may be in a variety of different configurations.
The diameter and height of solder balls 24 are important to the reliability and performance of the surface mounted electronic component. In general, it is desired to have a large diameter solder ball, providing a large stand-off between the surface mounted component and the PCB after reflow. This is so for at least two reasons. First, conventionally, larger solder balls are better able to absorb and withstand mechanical shock than smaller solder balls. Second, conventionally, larger solder balls are better able to absorb and withstand thermal stresses that are generated in solder balls due to the coefficient of thermal expansion (CTE) mismatch between the surface mounted electronic component and the PCB. Conventional solder balls may be about 0.3 mm in diameter. They may be generally spherical, and may flatten down to about 0.22 mm after they are reflowed.
While advantageous from a shock and CTE mismatch perspective, larger solder balls have the disadvantage of taking up valuable space along the height of an electronic component such as a BGA package. There is a drive to increase storage capacity and provide more semiconductor die in BGA and other flash memory packages. The height of such packages is often set by standards, and using an appreciable portion of that height for conventional solder balls reduces the space available for additional semiconductor die. It is known to surface mount LGA packages (having no solder balls), but underfill and/or mechanical adhesives are then required for shock and CTE mismatch which add to the cost and complexity of such designs.